1. Field of the Invention
The present invention relates to a variable valve timing control apparatus for an internal combustion engine, which apparatus includes an intermediate lock mechanism that locks a rotational phase of a camshaft relative to a crankshaft of the engine at an intermediate lock position. The rotational phase may be referred to as a “VCT phase”. and typically, the intermediate lock position is located between a full retard position and a full advance position of an adjustable range of the rotational phase.
2. Description of Related Art
in a conventional hydraulic variable valve timing device, as shown in JP-A-H9-324613 and JP-A-2001-159330, a lock position during engine stop is set at a generally middle phase within an adjustable range of a VCT phase such that the adjustable range of valve timing (VCT phase) is enlarged. In the above conventional art, the above intermediate lock position, at which the phase is locked during the engine stop, is set at a phase suitable for starting the engine. The engine is started while the VCT phase is at the intermediate lock position. Also, when oil pressure have been raised to a preferable pressure due to the increase of the engine rotational speed (oil pump rotational speed) after starting the engine, the lock is released such that valve timing (VCT phase) is feed-back controlled. In the above, the VCT phase is computed based on the pulse signals that are outputted synchronously with the engine rotation from rotation angle sensors (a cam angle sensor and a crank angle sensor). Thus, actuation oil pressure of the variable valve timing device is feed-back controlled such that the VCT phase, which has been released from the lock position, becomes a target VCT phase that is set in accordance with the engine operational state.
In the variable valve timing device having the above intermediate lock mechanism, a lock control is executed when the engine rotation speed is reduced equal to or less than a predetermined rotational speed in response to an engine stop request. More specifically, in the lock control, the intermediate lock mechanism is caused to lock the VCT phase. When the engine is operated in a substantially low rotation range, such as a case immediately before the stop of the engine rotation or a case of the start of the engine (during the cranking), the output pulses of the rotation angle sensors are not sharp enough, and thereby it is difficult to identify the edges of the pulses. As a result, it may be difficult to compute the VCT phase, and thus it may not be clear whether the VCT phase is locked at the intermediate lock position. Thus, even if the VCT phase is not locked at the intermediate lock position by failure during the stop of the engine, the failure in locking the VCT phase would not be identified erroneously. As a result, for example, in a case, where the VCT phase of the intake valve is located around the full advance position at the stopping of the engine, the engine accordingly has to be restarted under the above state during the start of the engine in the next operation. Thus, abnormal combustion, such as pre-ignition, is more likely to occur disadvantageously.